Petroleum coke is a very pure form of carbon and is the principal raw material used in the manufacture of baked carbon products and graphite products. Anthracite coal is also used as a raw material for certain classes of carbon products. Both petroleum coke and anthracite coal are also widely used in the manufacture of electrodes for the aluminum industry. Petroleum coke and anthracite coal, however, have a substantial volatile content, e.g., 5-15% and typically about 10% in the case of petroleum coke. Calcination is therefore necessary before the coke or coal can be used as a raw material in the manufacture of baked carbon products and graphite products or for other purposes. Calcination is accomplished by heating the carbonaceous material in a rotary kiln calciner, a vertical shaft calciner or an electric calciner to a temperature on the order of 1200.degree.-1800.degree. C., dependent upon the intended end use of the product. Most petroleum coke is calcined in a rotary kiln, and in electrode manufacture the calcination temperature of the petroleum coke is typically about 1250.degree. C. During the calcination step moisture, hydrocarbons, and other volatile components are removed and the density of the coke is increased.
The hot effluent gas from a rotary kiln calciner, contains hydrocarbon vapors and entrained carbonaceous solid or coke particles that cannot be discharged into the atmosphere under existing environmental restrictions. Instead, it is customary to mix the hot gas stream with combustion air in a thermal incinerator in order to effect combustion and removal of the hydrocarbon vapors and entrained carbonaceous solid or coke particles before the gas is discharged into the atmosphere. An additional pollution problem has been encountered, however, because the usual operating conditions in the thermal incinerator, including substantial excess oxygen and high combustion temperatures, are also conducive to the formation of oxides of nitrogen (NO.sub.x). Oxides of nitrogen are regarded as a major contributor to air pollution in many localities, and current government regulations require reduced emissions of this pollutant from stationary industrial sources.
Attempts have been made to control nitrogen oxide emissions by regulation of the thermal incinerator, for example, by injecting combustion air at various locations in the incinerator in order to reduce the excess oxygen available for the formation of nitrogen oxides. These attempts have generally been unsuccessful or unreliable because of the difficulties involved in mixing and distributing cold combustion air in the hot gas stream being incinerated.